Iridium clusters were prepared by chemisorption of [Ir-4(CO)(12)] on MgO powder supports that had been calcined at 300, 500, and 700 degrees C to vary the degree of hydroxylation. The initially adsorbed species, identified as tetrairidium carbonyl clusters by infrared and Raman spectroscopies, were decarbonylated by treatment in He at 300 degrees C and then treated in H-2 at 300 degrees C. The decarbonylated clusters at each stage were characterized by extended X-ray absorption fine structure (EXAFS) spectroscopy and X-ray absorption near-edge spectroscopy (XANES). After treatment in He, the clusters on each support nearly retained the tetrahedral metal frame of the [Ir-4(CO)(12)] precursor. After treatment in H-2 the clusters on MgO that had been calcined at 500 and at 700 degrees C still nearly retained this frame, whereas the clusters on MgO calcined at 300 degrees C underwent slight agglomeration. Ir-O interactions in the decarbonylated samples were indicated by the EXAFS data. In the family of samples treated in He but not H-2, the Ir-O bonding distance (approximately 2.1 Angstrom) decreased as the degree of dehydroxylation of the support increased. This trend, associated with an increasing loss of electron density from the iridium clusters, was also evidenced by an increase in the Ir white line area as the support dehydroxylation increased. Another Ir-O EXAFS contribution, near 2.6-2.7 Angstrom, is attributed to a nonbonding interaction influenced by cluster and support geometry. The Ir-O bonding distances increased after treatment of the samples in H-2, the increase being greatest for the MgO support that had been treated at 700 degrees C, from 2.07 to 2.15 Angstrom. The Ir-4/MgO clusters are among the most nearly uniform, stable, and well characterized supported metal clusters.